This invention relates to a device for the automatic measurement of the settlable solids content of a liquid, especially of a waste water/activated sludge mixture, with a light transparent vertical measuring tube which is filled with the liquid by a pump, and with a photoelectric scanning head for finding the position of the boundary layer of the deposited layer of solids (for instance, activated sludge).
Such a device is known from DE-OS No. 14 98 715. The design of this known device is expensive since at least two valves are required for proper functioning and also a cleaning device which must be run into the measuring tube to keep the inside wall of the measuring tube clean. The valves and the cleaning device require additional control means. In this known device, the liquid containing the solids is furthermore transported into the measuring tube by a pump which mixes, and thereby changes, the solids in the liquid. Also, the pump and the valves can be fouled by the solids contained in the liquid.